Minute rounded or pyriform parasites found within erythrocytes, or other circulating or endothelial cells of vertebrates, where they reproduce by merogony. The trophozoite stage separated from erythrocyte by single membrane. This distinguishes them from other blood parasites that usually have at least 2 membranes.
There is an apical complex with polar ring and rhopteries but without a conoid and usually without associated pellicular microtubules. They lack flagella and do not form either oocysts or spores.